Cant send to external email addresses via built in webmail..

I have an odd problem, no one can send emails to external email addresses if they use webmail that come with cpanel. I dont know why :s tried many diffrent emails it just not sending out. But if we used outlook and used our smtp works fine.. Incase you though i have unselected "Prevent the user 'nobody' from sending out mail to remote addresses (php and cgi scripts generally run as nobody if you are not using phpsuexec and suexec respectively.)" and it still does it.. any idea?

I'm having the same issue, email works fine via outlook and other local software, but nobody can send email via ANY of the webmail packages in cpanel, the email that gets sent back says "unrouteable mail domain"